Summary Deficits in goal-directed behavior are the hallmark of many neuropsychiatric diseases. The dorsomedial striatum (DMS) has emerged as a key mediator of goal-directed actions, serving as a critical node for integration of sensorimotor, motivational, and cognitive information. Nevertheless, the cellular mechanisms mediating these fundamental behaviors remain largely unclear. We have recently discovered that the low threshold spiking interneuron (LTSI) subtype within the DMS is a key regulator of early goal-directed actions. Performing the first in vivo imaging of this cell type during behavior, we uncovered robust reward-related activity that was down-regulated as animals learned an instrumental response task. Via subsequent neural circuit manipulations, we demonstrated that this reduction in LTSI activity could drive learning, while sustained activity slowed learning. In this proposal, we follow up these initial studies to explore the cellular and neural circuit mechanisms of these effects. We hypothesize that downregulation of LTSIs enhances the responsiveness of striatal circuits, a key step in driving behavior during early learning. We suggest LTSI downmodulation enhances striatal gain via two synergistic mechanisms: (1) increased local striatal dopamine levels and (2) enhanced corticostriatal input to SPNs via reductions in feedforward inhibition. Preliminary work demonstrates that LTSI inhibition can enhance striatal DA release, which may be an underlying mechanism driving enhanced acquisition. We will test whether LTSI inhibition enhances striatal DA during learning via calcium imaging of DA neuron terminals and virally-expressed DA sensors. To better understand the mechanism of this modulation, we will employ acute slice electrochemical measures of optically-evoked dopamine release during manipulation of LTSI activity. Finally, we will use circuit-targeted manipulations of DA neurons projecting to DMS to test whether enhanced striatal DA release is a mediator of the enhanced learning accompanying LTSI down regulation. Existing literature and preliminary data also suggest that LTSI are engaged in feed-forward control of SPN dendrites ? a key site for the integration of incoming neural signals. First, we describe both anatomically and electrophysiologically, how LTSIs integrate within key cortico- and thalamostriatal circuits. Next we use 2-photon microscopy to zoom into the level of SPN dendrites and synaptic spines, to understand how LTSIs regulate calcium signaling in these important compartments. In parallel, we explore long-term synaptic changes that accompany learning. Finally, we test whether LTSI-mediated gain changes within specific striatal circuits accounts for altered learning. When completed, these aims will provide our first glimpse into how striatal LTSIs gate learning, improving our understanding of the cellular mechanisms modulating goal-directed behavior.